1. Field of the Invention
The present invention relates to a duplex stainless steel useful for structural parts requiring strength and corrosion resistance, and more particularly, to a high manganese duplex stainless steel having excellent hot workability and a method for manufacturing the same.
2. Description of the Prior Art
Heretofore, duplex stainless steels have widely been used as basic materials in industrial equipment and structural parts requiring oxidation resistance and corrosion resistance. In particular, because 2205 type duplex stainless steels have higher corrosion resistance than austenite type stainless steels and are high in strength, they have been widely used in pipelines for chemical equipment, structural parts for dechlorination and desulfurization in power plants and the petrochemical industry, internal screw conveyors or bleaching reservoirs in the paper manufacturing industry, marine related equipment and the like. Recently, demands for duplex stainless steels have been increasing, because of the increase usages of dechlorination and desulfurization systems which are required in electric power stations or petrochemical equipment according to air pollution prevention policy. In addition to the above, these steels have been used as essential materials for air purification equipment in industrial waste incinerators.
Duplex stainless steels consist of a ferrite phase and an austenite phase, the ferrite phase improving strength and the austenite phase improving corrosion resistance. It is known in duplex stainless steels that pitting corrosion resistance and crevice corrosion resistance increase, resulting from the inclusion of Cr, Mo, W, and N in a basic Fe (R. N. Gunn, “Duplex Stainless Steels”, Woodhead Publishing Ltd., (1997)). After duplex stainless steels are subjected to casting or solution heat treatment, if they are not cooled at an appropriate rate, precipitates containing large amounts of Mo or W, including mainly sigma phase, are formed within the temperature range of 700 to 950° C. Furthermore, α′-phase forming zone is within the temperature range of 300 to 350° C. Precipitates formed at high or medium temperature improve the hardness of duplex stainless steels. However, there are problems in that room-temperature ductility and impact toughness drastically deteriorate and corrosion resistance drops.
Typically, commercial Mo-containing duplex stainless steels have a basic chemical composition of Fe-(21-23 wt %)Cr-(4.5-6.5 wt %)Ni-(2.5-3.5 wt %)Mo-(0.08-0.20 wt %)N, and further contain less than 2.0% of Mn and less than 0.03% of C (UNS31803 or SAF 2205). There exist SAF 2507 type duplex stainless steels with superior corrosion resistance, resulting from increasing contents of Cr and Mo in the 2205 type duplex stainless steels. They have a basic chemical composition of Fe-(24-26 wt %)Cr-(6-8 wt %)Ni-(3-5 wt %)Mo-(0.24-0.32 wt %)N and further contain less than 1.2% of Mn and less than 0.03% of C.
U.S. Pat. No. 4,657,606 discloses duplex stainless steels having a basic chemical composition of Fe-(23-27 wt %) Cr-(4-7 wt %) Ni-(2-4 wt %) Mo-(less than 0.08 wt %) C. It has been reported that if the content of Cu is limited to 1.1-3.0% and the content of Mn increases up to 5-7%, after solution heating and then cooling, the rapid formation of sigma- or α′-phase is inhibited, thereby room-temperature ductility being enhanced. However, these types of steels are poor in hot workability.
Meanwhile, many techniques have attempted to increase the content of Mn, considering the fact that Mn improves room-temperature ductility and increases solid solubility of nitrogen by replacing expensive Ni. U.S. Pat. No. 4,272,305 discloses that the content of N is defined as high as 0.35-0.6% and the content of Mn is increased up to 4-6%, resulting in increasing solid solubility of nitrogen in a duplex stainless steel of Fe-(22-28 wt %) Cr-(3.5-5.5 wt %) Ni-(1-3 wt %) Mo-(less than 0.1 wt %) C. However, this type of steel has a disadvantage in that, due to high content of nitrogen, castability and hot workability deteriorate. And, in U.S. Pat. No. 4,828,630 discloses that the content of Mn is increased up to 4.25-5.5%, thereby replacing expensive Ni and increasing solid solubility of nitrogen in a duplex stainless steel of Fe-(17-21.5 wt %) Cr-(1-4 wt %) Ni-(less than 2 wt %) Mo-(less than 0.07 wt %) C. However, this sort of steel has a problem in that the lower limit of Ni is low, capable of adversely influencing corrosion resistance. Japanese Patent Laid-Open Publication No. 9-31604 discloses that the content of Si is maintained to be high (2.5-4.0%), and in order to increase solid solubility of nitrogen, the content of Mn is increased to be 3-7% in a Mo—W containing duplex stainless steel. However, this type of steel has a problem in that, due to the excess Si, impact toughness deteriorates. Accordingly, it is difficult for this type of steel to be commercialized.
Meanwhile, there have been some attempts to add Mn to a Fe—Cr—Ni type austenite stainless steel known as 304 or 316 type stainless steel, in order to replace expensive Ni. However, as the added amount of Mn increases, hot workability deteriorates and thus satisfactory results are not obtained. This fact was reported in T.M. Bogdanova et al., Structure and Properties of Nonmagnetic Steels, Moscow, USSR, pp. 185-190, (1982). But, it has been reported that as a result of including Mn and S in 316L, 309S, and 310S type stainless steels, if the content of Mn is lower, a re-precipitation or segregation of S is easier, thereby deteriorating hot workability (S.C. Lee et al., 40th Mechanical Working and Steel Processing Conf., Pittsburgh, Pa., USA, pp. 959-966, (1998)).
Accordingly, in most commercial duplex stainless steels, to ensure hot workability, the content of Mn is limited to less than 2%. For example, U.S. Pat. No. 4,664,725 discloses that although hot workability is improved in a Ca/S ratio of greater than 1.5, the upper limit of Mn must be defined, since as addition of Mn increases, hot workability and corrosion resistance deteriorate.
As seen from the above, it is commonly regarded that as the content of Mn increases, hot workability deteriorates in duplex stainless steels. U.S. Pat. No. 4,101,347 proposes that the content of Mn should be limited to less than 2%, so as to prevent formation of sigma phase in the duplex stainless steel. This is supported by the fact that the content of Mn has been limited to less than 2% both in conventional Mo— or Mo—W containing duplex stainless steels.
Meanwhile, it is known that a Mo—W containing duplex stainless steel has an enhanced corrosion resistance. Therefore, recently, studies have been made on duplex stainless steels in which both Mo and W are added. For example, in a duplex stainless steel which was proposed by B. W. Oh et al., a part of Mo is replaced with W in a steel which contains less than 2.0% of Mn and 20-27% of Cr (Innovation of Stainless Steel, Florence, Italy, p. 359, (1993) or Korean Patent Application No. 94-3757). It is reported that a duplex stainless steel containing 1-4% of W and less than 1% of Mo has an improved corrosion resistance compared with that containing 2.78% of Mo. However, the above steel has an excessively low W and Mo content, and thus, the corrosion resistance is relatively decreased.
For another example, U.S. Pat. No. 5,298,093, filed by Sumitomo Metal Industries, Ltd., proposes that 2-4% of Mo and 1.5-5% of W are contained in a duplex stainless steel in which less than 1.5% of Mn and 23-27% of Cr are added. This steel is known to have high strength and excellent corrosion resistance. However, this steel is liable to crack during a hot rolling, and because it is a high-alloyed steel, the phase stability tends to be lowered, forming sigma phase, thereby deteriorating corrosion resistance and impact toughness. The W—Mo containing duplex stainless steel also has a problem in that hot workability is poor at the time of manufacturing finished product forms, including plate, wire, bar and pipe by hot working, similar to the above Mo-containing duplex stainless steel. As a result, a defective proportion of the products increases.
Similarly, U.S. Pat. No. 5,733,387 proposes that 1-2% of Mo and 2-5% of W are contained in a W—Mo containing duplex stainless steel in which less than 2.0% of Mn and 22-27% of Cr are added. However, this stainless steel still has little enhancement in hot workability, relative to the duplex stainless steel of U.S. Pat. No. 5,298,093.
In addition, U.S. Pat. No. 6,048,413 proposes a duplex stainless steel, in which less than 3.5% of Mn, 5.1-8% of Mo and less than 3% of W are contained. This steel is a high-alloyed duplex stainless steel and thus has the worst hot workability among the duplex stainless steels mentioned previously. Therefore, it is of limited utility for casting products. In addition, at the time of manufacturing products by casting, if cooling rate is slow (or if the size of a product is large), due to large quantities of Mo, formation of sigma phase is promoted, thereby deteriorating mechanical properties and corrosion resistance of the steel.
A conventional method for improving hot workability in duplex stainless steels involves adding Ce into the duplex stainless steels (J. L. Komi et al., Proc. of Int'l Conf. on Stainless Steel, ISIJ Tokyo, p 807, (1991) or U.S. Pat. No. 4,765,953). According to this method, the S content is lowered to 30 ppm, and Ce is added, so that the segregation of S is prevented, thereby improving the hot workability. However, in the case where hot workability is improved by adding rare earth elements such as Ce in large quantities, use of expensive Ce is unfavorable from an economic point of view. In addition to the above, the use of Ce has a problem in that strong oxidizing power of Ce causes clogging of nozzles upon continuous casting. As a result, the manufacture of billet or slab becomes hard. This duplex stainless steel does not contain W, but Mo.